In recent years, a liquid crystal display apparatus which is used for a television and a personal computer has been developed to have a large screen and a high definition. A source driver is required to drive a large load at a higher speed while suppressing consumed power. Also, many differential amplifiers have been mounted in the source driver. For this reason, the differential amplifier is required to operate with a high slew rate in a circuit area as small as possible.
For example, Japanese Patent Publication (JP 2001-156559A) discloses a circuit configuration and operation of the amplifier. As shown in FIG. 1, the high slew rate differential amplifier amplifies a differential input signal supplied to an amplifier positive input terminal INP31 and an amplifier negative input terminal INN31 and outputs the amplified result from an amplifier output terminal OUT31. When the amplifier is used as a source driver for a liquid crystal drive unit, the differential amplifier is used as a voltage follower-type amplifier having the gain of “1” by connecting the amplifier negative input terminal INN31 and the amplifier output terminal OUT31. When a voltage at the amplifier output terminal OUT31 is switched from a low voltage to a high voltage, a voltage at a node PG31 falls tentatively so as to turn on a transistor TP31. Thus, currents of the constant current sources ICS32 and ICS36 of an input differential stage are increased tentatively. Accordingly, the differential amplifier is set to a high slew rate. When the voltage at the amplifier output terminal OUT31 is switched from the high voltage to the low voltage, a voltage at the node NG31 is increased tentatively so as to turn on a transistor TN31. Thus, a current from a constant current source ICS35 is added to a current from a constant current source ICS31 and the current of the input differential stage increases tentatively. Accordingly, the differential amplifier is set to the high slew rate.
In this differential amplifier, when the voltage at the amplifier output terminal OUT is switched from the low voltage to the high voltage 31, the switching is sped up because the voltage at the node PG31 falls. However, because a time period for which the voltage at the node PG31 is set to the lower level is very long (tbp1 in (b) of FIG. 2: about 10 μs), the constant current of the differential input stage increases for the long time. For this reason, as shown in (d) of FIG. 2, a ringing waveform appears. Moreover, the differential input stage drags currents in a middle stage so that an oscillation operation is sometimes induced as an extraordinary operation.
The same operation as described above is carried out since the voltage at the node NG31 rises when the voltage at the amplifier output terminal OUT31 of the differential amplifier is switched from the high voltage to the low voltage. Moreover, because the differential amplifier returns to an ordinary operation after the switching operation, the gate voltages of the transistors TP31 and TN31 are such as the gate voltage of the transistor TP31≈VDD2-VTP and the gate voltage of the transistor TN31≈VTN. Because the transistors TP31 and TN31 must be set to an off state with such gate voltages left, the size (W/L) design of the transistors TP31 and TN31 becomes very difficult. Here, VTP and VTN are threshold voltages of the transistors TP31 and TN31, respectively.